The present invention relates to a process and apparatus for treating surfaces with laser light, particularly, though not necessarily, for the removal of foreign materials from substrate surfaces, e.g. removal of photoresist from semiconductor surfaces.
In the manufacturing of various products it is necessary to apply a layer of protective material on a surface, which must be removed after a specified manufacturing step has been concluded. An example of such process is the so-called xe2x80x9cmaskingxe2x80x9d, where a pattern is created on a surface using a layer of protective material illuminated through a mask, and the surface is then treated with a developer which removes material from the unmasked portions of the surface, therefore leaving a predetermined pattern. The surface is then treated by ion implantation or by etching agents, which introduce the implanted species into the unmasked portions of the surface, or removes material from unmasked portions. Once these processes are completed, the role of the protecting mask ends and it must be removed. The process is conventional and well known in the art, and is described, e.g., in U.S. Pat. No. 5,114,834.
U.S. Pat. No. 5,114,834 describes photoresist removal by ablation, using laser UV radiation, in an oxidizing environment, as described. The ablation process is caused by strong absorption of the laser pulse energy by the photoresist. The process is a localized ejection of the photoresist layer to the ambient gas, associated with a blast wave due to chemical bonds breaking in the photoresist and instant heating. The partly gasified and partly fragmented photoresist is blown upwards away from the surface, and instantly heats the ambient gas. Fast combustion of the ablation products occurs, due to the blast wave and may also be due to the photochemical reaction of the UV laser radiation and the process gases. The main essence of the process is laser ablation with combustion of the ablated photoresist which occurs in a reactive gas flowing through an irradiation zone. The combination of laser radiation and fast combustion provides instantaneous lowering of the ablation threshold of hard parts of the photoresist (side walls). The combusted ablation products are then removed by vacuum suction, or by gas sweeping leaving a completelyclean surface.
Copending patent applications Nos. IL 115931 and IL 119246 (attorney""s docket No. 4009) of the same applicant, the contents of which are incorporated herein by reference, describe and claim improved processes for laser removal of foreign materials from surfaces in the presence of reactive gases.
EP-A 0 200 089, corresponding to U.S. Pat. No. 4,670,639, discloses and claims a method for forming narrow metal-free strips in a metallic layer on a plastic sheet, from which capacitors are manufactured, which consists in moving the sheet relative to a plurality of pulse laser beams, arranged parallel to one another in a single plane parallel to the direction of translation of motion of the sheet, to successively generate metal-free areas which are overlapped to form a narrow, completely metal-free strip. The footprint of the laser beam is circular and therefore adjacent two prints overlap to form a wavy edge. The metal-free strips are spaced from one another to leave a metallized area between them. The plastic sheet is then divided into capacitor elements in correspondence to the metal-free strips, which constitute an insulating edge region of the metallization.
While reference is made in this specification to the ablation of photoresist from semiconductor wafers, this will be done for the sake of simplicity, and because it represents a well known and widely approached problem. It should be understood, however, that the invention described hereinafter is by no means limited to the stripping of photoresist from wafers, but it applies, mutatis mutandis, to many other applications, such as stripping and cleaning of photoresist from Flat Panel Displays (FPD) or removal of residues from different objects, such as lenses, semiconductor wafers, or photo-masks.
The laser treatment for the removal of foreign material from surfaces, developed by the art until now, have been found to present certain drawbacks.
One drawback consists in that, as the laser beam sweeps over the surface to be treated, the gases in contact with said surface become depleted of oxidizing components and therefore less effective for the treatment and the depleted gases diffuse beyond the zone which has undergone treatment to the neighboring zones, so that, as the laser beam sweeps over said surface, it encounters increasingly depleted and ineffective gases. Another drawback is that, as a zone of the body being treated is contacted by the laser beam and heated thereby, heat is transferred to some extent to neighboring zones, so that as the laser beam sweeps over the surface to be treated, it encounters zones increasingly heated by conduction, the temperature of which becomes increasingly higher than that required for the treatment. A further drawback is that relatively deep and narrow depressions of the surface to be treated, and particularly their side walls, are not reached by the laser beam as fully as desirable
It is a purpose of this invention to provide an improved process and apparatus for laser removal of foreign materials from surfaces.
It is another purpose of this invention to provide such a process and apparatus which eliminates the drawbacks of the known processes.
It is a further purpose of this invention to provide such a process and apparatus for laser treatment in the presence of reactive gases which permits to maintain an optimal composition of said gases over the treated zones.
It is a still further purpose of this invention to provide such a process and apparatus which avoids overheating of any portions of the treated body.
It is a still further purpose of this invention to provide such a method and apparatus that are based on simple and reliable optical means.
It is a still further purpose of this invention to provide such a method and apparatus that permits fully to treat the depressions of the treated surfaces, and in particular their side walls.
It is a still further purpose of this invention to provide a method and apparatus for the surface treatment of materials by laser pulses wherein which provide essentially stable conditions from pulse to pulse, without residual influence from previous pulses.
Other purposes and advantages of the invention will appear as the description proceeds.
The method of laser treatment of surfaces according to the invention comprises the steps of:
1) projecting onto the surface to be treated laser pulses defining a beam;
2) causing said beam and said surface to be treated to become relatively displaced in such a way that said beam will intersect at each number of pulses, preferably at each pulse, an area of said surface that is different from and non-adjacent to the area the beam intersected before said displacement; and
4) continuing to cause that the beam and said surface to be treated so to become relatively displaced, until all of said strips have thus been treated by the laser beam.
Preferably, the surface to be treated is ideally divided into a number of strips and said beam and said surface are caused to become relatively displaced in such a way that said beam will intersect at each number of pulses, preferably at each pulse, an area of said surface comprised in a strip that is different from and non-adjacent to the strip in which was comprised the area the beam intersected before said displacement.
The surface to be treated will be called hereinafter xe2x80x9cthe substrate surfacexe2x80x9d. The substrate surfaces are generally not plane or smooth, but have projections and/or recesses created e.g. in the masking process or in another process used for imparting to said surface the desired pattern and properties. The expression xe2x80x9cideal surface corresponding to the surface to be treatedxe2x80x9d or xe2x80x9cto the substrate surfacexe2x80x9d, or, briefly xe2x80x9cideal surfacexe2x80x9d, as used in this specification and claims, means the plane or smooth surface that would result from the smoothing out of said projections and/or recesses, or, in other words, the surface that existed before said masking or other process was carried out and would still exist had it not been carried out.
Preferably, the laser beam impinges on the ideal surface at an angle to the perpendicular to said surface.
In a preferred form of the invention, the relative displacement of the beam and said substrate surface comprises two displacement components: a switch of the beam from strip to strip and a scanning motion perpendicular of said strips. More preferably, the switch occurs between non-adjacent strips, and the scanning is continuous. Still more preferably, the switch is synchronous with the laser pulses and the scanning occurs at such a rate as to cause the laser beam to impinge a number of times on each point of substrate surface, said number of times being such as to cause the beam to apply to each such point a predetermined amount of energy.
The center lines of the strips can have any configuration, e.g. they may be circular, elliptical, spiral-shaped or have more complicated shapes. In a preferred embodiment of the invention, however, they are rectilinear.
While the last mentioned embodiment of the invention will be particularly described hereinafter, it should be understood that it constitutes only an example and that its description is not limitative in any way.
The relative scanning motion of the laser beam and of the substrate surface can be produced by displacing either the beam or the surface itself.
Preferably, the relative switches of the laser beam with respect to the substrate surface are synchronized with the laser pulses, generally so as to produce one switch for each passage from one pulse to the next.
In a preferred embodiment of the invention, the laser beam is firstly directed at any given portion of the substrate surface at a slant to one side of the perpendicular to the ideal surface and, after at least two switches, is directed to said portion or substantially thereto, at a second slant opposite to said first one. It can be said that, if xcex1 is the absolute value of the angle between the laser beam and said perpendicular, the beam is slanted alternatively by +xcex1 and by xe2x88x92xcex1. xcex1 is chosen as a function of the expected xe2x80x9cAspect Ratioxe2x80x9d, which is the ratio of depth to width of the depressions of the surface to be treated. Angle xcex1 is higher if the Aspect Ratio is lower, and vice versa. Indicatively, xcex1 may be the angle the cotangent of which is equal to the expected Aspect Ratio. While in the embodiments described it is assumed to be about 20xc2x0,it could have different values and, e.g., could be as high as 60xc2x0 or even higher. When said angle is 20xc2x0, the expected Aspect ratio is close to 3 (2.777).
When the invention is used in laser stripping processes which involve treatment is an oxidizing atmosphere, as hereinbefore recalled, the process comprises feeding a reactive gas to flow over the substrate surface. In this case, preferably, the gas flows parallel to the strips in which the substrate surface is ideally divided.
The apparatus for carrying out the said method comprises, in combination with means for generating a laser beam constituted by a succession of pulses, means for deviating the laser beam and directing it to impinge successively on different, non-adjacent zones of a surface to be treated, according to a predetermined displacement succession.
The aforesaid laser beam generating, deviating and directing means, together, will be called sometimes xe2x80x9cthe laser beam apparatusxe2x80x9d.
In a preferred form of the invention, the substrate surface is ideally divided into a number of strips, and the means for deviating the laser comprise means for switching the beam from strip to strip and means for generating a relative scanning motion perpendicular of said strips. More preferably, the switching means switch between non-adjacent strips, and the scanning motion is continuous. Still more preferably, the switching means are synchronous with the laser pulses and the scanning motion has such a speed occurs as to cause the laser beam to impinge a number of times on each point of substrate surface, said number of times being such as to cause the beam to apply to each such point a predetermined amount of energy.
The laser beain can be deviated e.g. by means of prisms or of mirrors or of a combination of prisms and mirrors, and these and all other deviating means, collectively called xe2x80x9cthe beam deviating meansxe2x80x9d, are included in the scope of the invention.
Preferably, the laser beam deviating and directing means are such and are so synchronized with said succession of laser pulses that the laser beam impinges on each zone of the substrate surface twice and at opposite slants thereto.
A preferred form of the apparatus for carrying out the method of the invention comprises, in combination with means for generating a laser beam:
Ixe2x80x94a movable support carrying a number of laser beam upper deviating means, such as upper prisms (or mirrors);
IIxe2x80x94means for actuating said support, particularly to rotate, to move at a controlled speed;
IIIxe2x80x94means for directing the laser beam to an active position, that is successively occupied by each of said upper deviating means, as the support moves;
IVxe2x80x94a stationary laser beam deviating means assembly, comprising a number of pairs of lower deviating means, such as prisms (or mirrors);
Vxe2x80x94said upper deviating means being so shaped or oriented that each one of them directs said laser beam to a different one of said lower deviating means;
VIxe2x80x94said lower deviating means being so shaped (or oriented) as to direct said deviated laser beam towards the surface to be treated;
VIIxe2x80x94the two lower deviating means of each said pair directing the twice deviated laser beam to the same zone of said surface at opposite slants thereto; and
VIIxe2x80x94means for relatively displacing said beam deviating means and said surface to be treated.
Preferably, said upper and lower deviating means are prisms or mirrors; and, more preferably, said means for actuating said support are so controlled that the frequency with which a prism (or mirror) occupies said active position is equal to the frequency of the laser pulses.
In a further preferred form of the apparatus, said support is a rotating wheel having a number of upper prisms (or mirrors) at its periphery, and the means for directing the laser beam to an active position successively occupied by each of said upper deviating means, as the support moves, are means for directing the laser beam to a position at said wheel periphery wherein it impinges successively on each of said prisms (or mirrors) as the wheel rotates.
Therefore, in such a form of the apparatus for carrying out the method of the invention comprises, in combination with means for generating a laser beam:
Ixe2x80x94a rotating wheel having a number of upper beam deviating means at its periphery;
IIxe2x80x94means for directing the laser beam to a position at said wheel periphery wherein it impinges successively on each of said beam deviating means as the wheel rotates;
IIIxe2x80x94a stationary beam deviating means assembly, comprising a number of pairs of lower beam deviating means;
IVxe2x80x94said upper beam deviating means being so shaped or oriented that each one of them refracts (or reflects) said laser beam to a different one of said lower beam deviating means;
Vxe2x80x94said lower beam deviating means being so shaped (or oriented) as to refract (or reflect) said refracted (or reflected) laser beam towards the surface to be treated;
VIxe2x80x94the two lower beam deviating means of each said pair directing the twice refracted (or reflected) laser beam to the same elementary surface of said surface at opposite slants thereto; and
VIIxe2x80x94means for relatively displacing said wheel, upper prisms and lower beam deviating means and said substrate surface.
When the invention is used in laser stripping processes, such as those hereinbefore recalled, the apparatus also comprises a device, generally a cassette, for holding the body of which the substrate surface is a part (which can be briefly called xe2x80x9ctreated bodyxe2x80x9d), e.g. a wafer, and providing a reactive gas atmosphere, and the means for relatively displacing the laser beam and the substrate surface may be means for displacing said device for holding said treated body.
The method of the invention, however, could be implemented by means of apparatus different from that herein described and such implementations are within the scope of the invention and of the claims appended hereto.